This invention relates to engine manufacture and, more particularly, to a method of providing enlarged cylinder bores in a cylinder block where bore enlargement is constrained on one side of the cylinder.
It is common in the art of engine design and manufacture to provide a cylinder block with one or more cylinders arranged in one or more banks wherein the cylinders are centered on axes which intersect the longitudinal centerline or axis of the engine crankshaft carried in a lower portion of the block of crankcase. When a new engine family is designed, the cylinder walls are sometimes made thicker than absolutely necessary to leave room for a later increase in cylinder displacement by a suitable increase in the size of the cylinder bore. At some point, however, further increase in the cylinder bore to one side of the cylinders may be limited by reaching of a minimum cylinder wall thickness due to the initial design features.
For example, in a traditional overhead valve (OHV) engine, the valves may be actuated through pushrods from a camshaft located in a tunnel or series of bores positioned adjacent one side of the cylinders in a cylinder bank. In a V-type engine, the camshaft bore is conventionally located between the V-angled banks of cylinders. Thus, at some point, enlargement of the cylinder bores around the original cylinder axis may be constrained by reduction of the cylinder wall thickness at the location of the camshaft bore to a minimum wall thickness dimension, thereby limiting the ability of the engine to have the cylinder bore diameter further increased.
The present invention provides a method of enlarging the cylinder bores of a family of internal combustion engines wherein the cylinders are constrained against enlargement toward one side of the cylinders, for example, by the location of a camshaft bore or coolant jacket. If the engine construction and design conditions allow, some enlargement of the cylinder bores may be possible by offsetting the cylinder axis away from the constrained side of the cylinder and enlarging the bore by an amount permitted by the engine design. Preferably, the cylinder bore enlargement is limited to a dimension which may be provided without requiring a change in the cylinder head bolt pattern. Thus, major redesign or modification of the engine manufacturing equipment may be limited. Also, the amount of enlargement is dependent upon locations of other portions of the coolant jacket in the engine cylinder block. Nevertheless, where permitted by the design of the engine family, some significant bore enlargement may be possible without resorting to radical changes in the engine components and tooling required for manufacturing the family of engines.
In an example, the bores of the cylinders in the cylinder block of a conventional V-type OHV engine may have been designed, or previously enlarged, so that the camshaft bore constrains the cylinders against further enlargement toward the inside of the cylinder banks. An enlargement of only 3.0 mm in the bore would provide a substantial increase in the displacement of the enlarged cylinders without changing the stroke of the pistons. The engine design may permit an increase in the cylinder diameter by offsetting a new cylinder axis outward, away from the inner side of the cylinders by one half of the enlarged cylinder bore, or a 1.5 mm offset. The cylinders would then be bored on the new axis so that the outer wall of the enlarged cylinders would move outward 3.0 mm while the inner wall remains in the original position with its minimum thickness. In this instance, the change may be possible without modifying the head bolt pattern provided by the current engine tooling and without dropping below a wall thickness between other parts of the cylinders and the water jacket or other adjacent cavities.
Movement outward of the cylinder axes causes an offset of the cylinders so that the cylinder axes do not intersect the axis of the camshaft, as is conventional in engines. Instead, in a V engine of the invention, the cylinder axes of the opposite banks intersect a reference line below and parallel to the crankshaft axis by a small dimension. The invention provides the benefit of increased engine displacement without a requirement for major retooling of an engine line where conventional methods of bore enlargement could not be utilized. The relatively small offset of the cylinder axes could have slightly different effects on the two banks of cylinders, as far as piston thrust loads and possibly ignition timing are concerned. However, the differences should not exceed reasonable limits and may have advantages for deactivated cylinder operation where one bank of cylinders is deactivated for operation in a lower power range.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.